feat(hints): support minimum optimization levels

RFC 3924 lets a package request a numeric optimization floor without
overriding an application's profile choices. Apply the hint after
profile and built-in host defaults but before explicit package
and build overrides, while leaving the unordered size levels unchanged.

Signed-off-by: 0xPoe <poe.liu@pm.me>

Author: 0xPoe

Cargo.lock

@@ -36,7 +36,7 @@ cargo-platform = { path = "crates/cargo-platform", version = "0.3.3" }
 cargo-test-macro = { version = "0.4.15", path = "crates/cargo-test-macro" }
 cargo-test-support = { version = "0.11.4", path = "crates/cargo-test-support" }
 cargo-util = { version = "0.2.33", path = "crates/cargo-util" }
-cargo-util-schemas = { version = "0.14.4", path = "crates/cargo-util-schemas" }
+cargo-util-schemas = { version = "0.15.0", path = "crates/cargo-util-schemas" }
 cargo-util-terminal = { version = "0.1.3", path = "crates/cargo-util-terminal" }
 cargo_metadata = "0.23.1"
 clap = "4.6.0"

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cargo-util-schemas"
-version = "0.14.4"
+version = "0.15.0"
 rust-version = "1.98"  # MSRV:1
 edition.workspace = true
 license.workspace = true

crates/cargo-util-schemas/Cargo.toml

@@ -1030,6 +1030,16 @@
     "Hints": {
       "type": "object",
       "properties": {
+        "min-opt-level": {
+          "anyOf": [
+            {
+              "$ref": "#/$defs/TomlValue"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
         "mostly-unused": {
           "anyOf": [
             {
@@ -1444,7 +1454,11 @@
       "type": "string"
     },
     "TomlDebugInfo": {
-      "type": ["string", "integer", "boolean"],
+      "type": [
+        "string",
+        "integer",
+        "boolean"
+      ],
       "enum": [
         "none",
         "line-directives-only",

crates/cargo-util-schemas/manifest.schema.json

@@ -1705,6 +1705,11 @@ pub enum TomlLintLevel {
 #[serde(rename_all = "kebab-case")]
 #[cfg_attr(feature = "unstable-schema", derive(schemars::JsonSchema))]
 pub struct Hints {
+    #[cfg_attr(
+        feature = "unstable-schema",
+        schemars(with = "Option<TomlValueWrapper>")
+    )]
+    pub min_opt_level: Option<toml::Value>,
     #[cfg_attr(
         feature = "unstable-schema",
         schemars(with = "Option<TomlValueWrapper>")

crates/cargo-util-schemas/src/manifest/mod.rs

@@ -127,6 +127,7 @@ pub fn generate_std_roots(
     interner: &UnitInterner,
     profiles: &Profiles,
     target_data: &RustcTargetData<'_>,
+    hint_min_opt_level: bool,
 ) -> CargoResult<HashMap<CompileKind, Vec<Unit>>> {
     // Generate a map of Units for each kind requested.
     let mut ret = HashMap::default();
@@ -149,6 +150,7 @@ pub fn generate_std_roots(
             interner,
             profiles,
             target_data,
+            hint_min_opt_level,
         )?;
     }
 
@@ -167,6 +169,7 @@ fn generate_roots(
     interner: &UnitInterner,
     profiles: &Profiles,
     target_data: &RustcTargetData<'_>,
+    hint_min_opt_level: bool,
 ) -> CargoResult<()> {
     let std_ids = std_crates(crates, default, units)
         .iter()
@@ -191,6 +194,8 @@ fn generate_roots(
             let unit_for = UnitFor::new_normal(kind);
             let profile = profiles.get_profile(
                 pkg.package_id(),
+                pkg.hints(),
+                hint_min_opt_level,
                 /*is_member*/ false,
                 /*is_local*/ false,
                 unit_for,

src/compiler/standard_lib.rs

@@ -867,6 +867,8 @@ fn new_unit_dep(
     let is_local = pkg.package_id().source_id().is_path() && !state.is_std;
     let profile = state.profiles.get_profile(
         pkg.package_id(),
+        pkg.hints(),
+        state.gctx.cli_unstable().hint_min_opt_level,
         state.ws.is_member(pkg),
         is_local,
         unit_for,

src/compiler/unit_dependencies.rs

@@ -20,7 +20,7 @@
 //! - TOML syntax or manifest schema: [`passes::emit_parse_diagnostics`], [`rules::PARSE_PASS_RULES`]
 //! - Lockfile
 //!   - May be overly broad for what dependencies are checked
-//! - Pre-build unit graph
+//! - Pre-build unit graph: [`rules::min_opt_level_hint::diagnose`]
 //!   - Tailored to a specific configuration (features, targets) but requires users to enumerate every configuration
 //! - Post-build unit graph: [`rules::unused_dependencies::lint_build_results`]
 //!   - Slow feedback cycle since a build needs to happen

src/diagnostics/mod.rs

@@ -0,0 +1,47 @@
+use crate::CargoResult;
+use crate::compiler::BuildContext;
+use crate::workspace::profiles::{MinOptLevelHintError, parse_min_opt_level_hint};
+
+/// Emits diagnostics for `hints.min-opt-level` once per package selected for compilation.
+#[tracing::instrument(skip_all)]
+pub(crate) fn diagnose(bcx: &BuildContext<'_, '_>) -> CargoResult<()> {
+    let gctx = bcx.gctx;
+    let mut packages = bcx
+        .unit_graph
+        .keys()
+        .filter(|unit| !unit.skip_non_compile_time_dep && unit.show_warnings(gctx))
+        .map(|unit| unit.pkg.clone())
+        .collect::<Vec<_>>();
+    packages.sort_by_key(|pkg| pkg.package_id());
+    packages.dedup_by_key(|pkg| pkg.package_id());
+
+    for pkg in packages {
+        let warn = |message: &str| {
+            gctx.shell()
+                .warn(format!("{}@{}: {message}", pkg.name(), pkg.version()))
+        };
+        let min_opt_level = match parse_min_opt_level_hint(
+            pkg.hints().and_then(|hints| hints.min_opt_level.as_ref()),
+        ) {
+            Ok(level) => level,
+            Err(MinOptLevelHintError::OutOfRange(level)) => {
+                warn(&format!(
+                    "ignoring unsupported value ({level}) for 'hints.min-opt-level', which only supports integers from 0 to 3"
+                ))?;
+                None
+            }
+            Err(MinOptLevelHintError::WrongType(value_type)) => {
+                warn(&format!(
+                    "ignoring unsupported value type ({value_type}) for 'hints.min-opt-level', which expects an integer"
+                ))?;
+                None
+            }
+        };
+
+        if matches!(min_opt_level, Some(1..=3)) && !gctx.cli_unstable().hint_min_opt_level {
+            warn("ignoring 'hints.min-opt-level', pass `-Zhint-min-opt-level` to enable it")?;
+        }
+    }
+
+    Ok(())
+}

src/diagnostics/rules/min_opt_level_hint.rs

@@ -2,6 +2,7 @@ mod blanket_hint_mostly_unused;
 mod deferred_parse_diagnostics;
 mod im_a_teapot;
 mod manual_readme;
+pub mod min_opt_level_hint;
 mod missing_lints_features;
 mod missing_lints_inheritance;
 mod non_kebab_case_bins;